const(Object)ref is here!

[Michel Fortin]

After a recent discussion on this list about tail-const class 
references, it became rather clear that someone interested would have 
to implement the thing if we were to have it. So I did it. See 
enhancement request 5325 for an explanation and a patch.
<http://d.puremagic.com/issues/show_bug.cgi?id=5325>

Let's hope Walter likes my patch.

In the tail-const thread, I proposed the challenge of making this code 
work when an array of const object is passed as an argument:

	T[] giveMeASortedArray(alias Predicate, T)(T[] t) {
		// creating new array of the same length but with assignable elements
		auto copy = new Unqual!(typeof(t[0]))[t.length];
		foreach (index, value; t)
			copy[index] = value;
	
		// sorting the copy
		sort!(Predicate)(copy);
		return copy;
	}

Well, with the patch I made it now works!... irrespective of the type 
attribute (const,immutable,shared,inout). The only modification 
required to Phobos is to make to!string() compile when passed a 
const(Object) because somehow 'sort' requires that.

Here is how the function is invoked:

	void main() {
		int*[] a = giveMeASortedArray!("a < b")(new int*[12]);
		Object[] b = giveMeASortedArray!("a < b")(new Object[12]);
	
		const(int*)[] c = giveMeASortedArray!("a < b")(new const(int*)[12]);
		const(Object)[] d = giveMeASortedArray!("cast(void*)a < 
cast(void*)b")(new const(Object)[12]);
	}

See the strange predicate for the const(Object) version? That's because 
opCmp() in Object doesn't work with const.

Two minor modifications are required in Phobos to make the above 
compile. First, to!string(const(Object)) needs an implementation 
because somehow 'sort' requires it. Also template std.traits.isMutable 
needed an adjustment so that swap()'s template constrains are satisfied.


-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/